Significance Of Transformer Oil Monitoring

It is of vital importance to monitor the condition of transformer oil. Periodic testing of transformer oil is necessary to ensure safe, economical, trouble free and undisturbed power supply... - V. G. Patel, Jay B. Thakar

The powerhouses may be far away from the load centers as in the case of hydro powerhouses – or they may be in the midst of populated areas as in the case of steam powerhouses. The transmission network is inevitable. Long and high voltage transmission lines are necessary to transmit huge blocks of power from the sources of generation to the load centers, to interconnect power-houses for increased reliability of supply, greater system stability and lesser standby power plant and hence cheaper electric energy. In between the powerhouses and receiving substations, and the ultimate consumers, no. of transformers having capacity of hundreds of MVA to hundreds of KVA or even less (distribution transformers) are installed. At sending end step-up transformers and at receiving end step-down transformers are installed. Anything going wrong anywhere with any transformer, a very huge area will be affected. Therefore, it becomes very important to monitor the transformer health, which is responsible for uninterrupted power supply. The transmission lines and circuit breakers also contribute for the same, but practically the weak link is transformer. As we all know, the petroleum-based mineral insulating oil is used in transformer as a coolant and as a dielectric medium, i.e., the oil is used for giving cooling effect as well as for providing insulation. Supposing everything else is healthy for a transformer except oil, which is not a part of workman-ship for manufacturing a transformer, the contaminated oil or the deteriorated oil can lead to tripping of transformer or failure of transformer or in the worst case bulging / bursting of transformer. So, it is of vital importance to monitor the condition of transformer oil. Periodic testing of transformer oil is necessary to ensure safe, economical, trouble free and undisturbed power supply. Oil in a transformer serves the double purpose of insulation and cooling. All oils are generally good insulators, but animal and vegetable oils are not suitable for use in transformers because these tend to form destructive fatty acids that attack fibrous materials which are normally used as winding insulation. The oil which is extensively used is called transformer oil and is a mineral oil obtained from fractional distillation of crude petroleum.

The insulating oil used for transformers meet the following requirements

a. Provide a high electric strength.
b. Permit good transfer of heat.
c. Have low specific gravity – In oil of low specific gravity particles, which have become suspended in the oil will settle down on the bottom of the tank more readily and at a faster rate, a property aiding the oil in retaining its homogeneity.
d. Have a low viscosity – Oil with low viscosity, i.e., having greater fluidity, will cool Transformers at a much better rate.
e. Have low pour point – Oil with low pour point will cease to flow only at low temperatures.
f. Have a high flash point.
The flash point characterizes its tendency to evaporate. The lower the flash point the greater the oil will tend to vaporize. When oil vaporizes, it loses in volume, its viscosity rises, and an explosive mixture may be formed with the air above the oil.
g. Not attack insulating materials and structural materials.
h. Have chemical stability to ensure lifelong service.

Various national and international specifications have been issued on insulating oils for transformers to meet the above requirements.

The permittivity of transformer oil is 2.5. As per Indian Standard specification, the breakdown strength of new transformer oil when treated must be at least 50 KV RMS. when measured with the help of two spherical electrodes of 12.5 mm diameter and with a gap spacing of 4 mm. However, the breakdown strength is greatly reduced due to the presence of impurities like moisture, gas bubbles, solid particles etc. A major disadvantage of the transformer oil is its sludging. Sludging means slow formation of semi-solid hydrocarbons due to heat and oxidation.

The breakdown voltage test for insulating oil…

Sludge is formed more quickly in the presence of bright copper surfaces. These sludges get deposited on the transformer windings, tank walls and at the bottom of the tank. Being bad conductor of heat, sludges greatly reduce the heat transfer from the windings to the oil and increase the temperature of the windings. Moreover, sludges block the cooling tubes and further increase the temperature of the transformer and may make the transformer unusable due to overheating. Sometimes to prevent sludging, certain chemicals called inhibitors are added to the transformer oil.

Inspite of many excellent properties, as it is inflammable (flash point-140ºC; due to fire hazard, C.B. manufacturers discontinued manufacture of BOCBs). To avoid the risk of fire and explosion, synthetic transformer oils are developed. Manufacturers started manufacturing other oil (liquid), which is fire retardant having flash point more than 250ºC. It is a chemical fluid. Its trade name is ASKAREL; generic name is PCB (Polychlorinated Biphenyl). Its dielectric strength is greater than mineral oil and its permittivity is 4.5, which is nearly equal to those of solid insulating materials.

However, a major disadvantage is that under the influence of power areas, this gets decomposed and forms hydrochloric acid, which is a greatly corrosive and toxic substance.

Though it is costly and not having comparative excellent properties like transformer oil, for years together the industries were using it – particularly PAPER industries – because of its high flash point.

But since last few years when it has been known that PCB is toxic which is very dangerous for human health, its use is stopped. Except India, all countries of the world have BANNED use of PCB and Govt. has passed orders to industries to replace ASKAREL liquid of transformers by conventional transformer oil in phased manner within a specified period of few years. Even in India, supply of transformers with ASKAREL liquid is discontinued. Government has allowed the industries the use of PCB only for topping up purpose in existing transformers. Multi-national companies purchase transformer oil only after confirming in writing (obtaining certificate) from oil manufacturers that it is not containing PCB.

Oil deterioration

Deterioration of oil begins from the moment it is filled in the transformer due to ageing and oxidation. The oil produces undesirable products like acids, sludges, moisture etc. The transformer oil is liable to deteriorate under normal operating conditions.

In some applications, oil is in contact with air. It is hence prone to oxidize, accelerated by the presence of catalysts.

Consequently, the oil darkens in colour and the acid in it begins to increase, thereby increasing sludge and consequently causing other electrical properties such as dissipation factor tan d to increase, ultimately hindering the life of transformer.

Transformer oil is petroleum based (Hydrocarbon) mineral oil. Till end of 1968, total requirement of transformer oil in the country was imported. 1968 was the golden year when first Indigenous transformer oil was produced in our country. Oil manufacturers purchase raw oil from refineries, which abstract it from crude oil. Though it is mineral oil, as more than 90% of oil is used in transformers and hence the name ‘Transformer Oil.’ It is like Dalda Ghee. Dalda is a brand name of vanaspati ghee. Dalda has become such a popular that almost all the shopkeepers will join word Dalda with other brand name. (Arun Dalda Ghee, Sagar Dalda Ghee, etc).

Three type of transformer oils are available:

Paraffinic base
Aromatic base
Naphthenic base.

Naphthenic base transformer oil is more stable compared to others. Today we are going to discuss transformer oil terminology and then testing of transformer oil in details.

Transformer oil terminology

According to Practicing Engineers, jargon of wordings – IS 335, PART – III, CLAUSE – B (i);

Basic Function of Oil & Engineers’ concern – Controlled temperature – efficient cooling.

  • Insulating property: –Should not interfere with any part. Therefore, it must be completely free from dirt, dust, fibers, moisture and other solid matters.
    • Dust, dirt and moisture are enemies of electricity, remove them.
    • Should be stable against oxidation at working temperature of 90 deg. C
    • Right viscosity and thermal conductivity to be an efficient coolant.
    • Should not have a tendency to dissolve any matter.
    • Operating Engineer would perhaps be knowing standard values for these terms but often wonders what it means in practical life.
    • Density of oil is 0.89 gr/cm3 at 29.5 °C.

Specification of insulating oil

Certain properties of insulating oil are very important.

These are:

  • Low viscosity.
    • Low pour point.
    • High flash point.
    • Excellent chemical stability.
    • High electrical strength.

There are also some other properties which might be less important, but for which it would nonetheless be desirable to have some say in their determination.

These include:

  • High specific heat.
    • High thermal conductivity.
    • Good impulse strength.
    • High or low permittivity, depending on intended use.
    • High or low gas absorbing, depending on intended use.
    • Low solvent power.
    • Low density.
    • Good arc quenching properties.
    • Non-toxic.

Static electrification of transformer oil

When insulating oil flows through insulation ducts in power transformers, charge separation occurs at the interface of paper and oil. As a result the insulating oil and the surface of the solid materials become charged and dielectric breakdown is possible within the oil or at the interface of insulating materials. At least dozen field failures of large forced oil cooled power transformers have been found. These failures were not attributed to any specific transformer design or type of oil.

Useful information about the trend of change in the oil (can be known from its colour and odor):

  • Cloudiness in oil may be due to suspended moisture or sediments such as iron oxide or sludge.
    • Dark Brown coloured oil may indicate the presence of dissolved asphaltenes.
    • A Green colour indicates the presence of dissolved copper compounds and a rapid deterioration of oil may be expected.
    • Acrid acid smell indicates the presence of volatile acids, which can cause corrosion.
    • The mineral hydrocarbon oil obtained from raw petroleum degrades through various processes.

Oxidation process begins when a small quantity of oil combines chemically with the dissolved oxygen in the oil (from air) resulting the formation of traces of organic acids. Carbon dioxide (CO2) is liberated predominantly during oxidation.

The direct breakdown of oil by arcing results in cracking of oil.

The aromatic content of the oil breaks down into simple hydrocarbons gas and hydrogen.

Acetylene and Methane are major constituents. Other hydrocarbon gases may also be liberated due to cracking.

OIL should withstand

  1. 30kV for one minute for oil in drum.
    ii. 40kV for one minute for oil in equipment. Short circuit current should be more than 20 m.a. above 15 kV. It is limited to 1.0 Amp max. In both the cases, withstand time is one minute.

D.S. of oil should be increased to 40 kV for one minute by filtration before transferring to electrical equipment.

Permitted water content (in PPM)

Generally a sample of 300–400 ml of transformer oil is taken. The electrodes may be of brass, copper, bronze or stainless steel. Diameter should be 12.7mm or 36 mm ± 0.02mm. These should be immersed in oil to be tested at 40 mm oil depth.

If stirrer is not provided with testing kit, after collecting sample of oil, the jar is put aside for at- least 20 minutes during which air bubbles mixed with oil will come on the top surface and oil will give correct B.D. strength.

The voltage rise of the apparatus should not be more than 2 KV per Sec. The auto trip relay acts in 0.02 sec after breakdown.

Generally 6 readings are taken at 1min. interval and the average of 6 is taken to give the dielectric strength of oil.

Relevant is

IS 335-1971-1983-1983A-1983B-1983E -1983F
IS 1448-1967-1970-1976-1977
IS 6103-1971
IS 6104-1971
IS 6162-1971
IS 6262-1971
IS 12177-1971-1987

Oil alternatives

  • Large transformers to be used indoors must use a nonflammable liquid or be Dry Type, i.e., having no fluid.
    • Prior to about 1970, polychlorinated biphenyl (PCB) was often used as a dielectric fluid since it was not flammable. However, under incomplete combustion, PCBs can form highly toxic products, furans, etc. Due to the stability of PCB and its environmental accumulation, it has not been permitted in new equipment since late 1960’s in the United States.
    • Today, nontoxic, stable silicone-based or fluorinated hydrocarbons may be used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used, but all fire-resistant fluids have various drawbacks in performance, cost, or toxicity compared with mineral oil.

Oil Test 


Moisture present in oil – free, suspended or dissolved affects performance of transformer. Dielectric strength of oil is the voltage in kV at which breakdown occurs in transformer oil. Test procedures and value (As per IS 6712)…

Synthetic oil


Tips for trans. oil health

  • Highest temperature should be recorded.
    • Allowable temperature rise should not be crossed.
    • Never compromise with quality.
    • Keep temperature well within the limit.
    • Schedule should be prepared for filtering.
    • Team work will help a lot.
    • On line monitoring should be adopted.
    • Attempt should be made for opportunity maintenance.
    • Limit for temperature rise should be monitored.
    • Loading on transformer should be monitored.

If you want to share any thought or feedback on this article then please leave a comment below.

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